1. Field of the Invention
The present invention relates to a molded semiconductor device, and more particularly to a molded semiconductor device for use in a high-frequency band.
2. Description of the Related Art
Known molded semiconductor devices inventions are disclosed, for example, in U.S. Pat. No. 5,057,805 and Japanese Patent Laid-open No. 85155/94 (the inventor Kazuyoshi Uemura). According to the disclosed inventions, metal members connected to a semiconductor chip by metal wires or the like by a bonding process allow electric connection between an external circuit extending from inside the resin mold to the outside and the semiconductor chip in the resin mold.
FIGS. 1(a) and 1(b) of the accompanying drawings show, in plan and fragmentary cross section of a high-frequency signal transmission line at enlarged scale, respectively, the conventional molded semiconductor device disclosed in U.S. Pat. No. 5,057,805.
As shown in FIG. 1(a), a semiconductor chip 3 mounted on a first metal member 2 as a ground conductor, and second metal members 1 as a central conductor serve as terminals for connection to an external circuit. The first metal member 2 has portions disposed in sandwiching relation to the second metal members 1. The second metal members 1 and the first metal member 2 jointly constitute transmission lines or input/output terminals 8 for high-frequency signals. Metal wires 4 connect the semiconductor chip 3 and the second metal members 1 to each other by bonding, thus connecting the semiconductor chip 3 to the external circuit. Metal wires 5 connect the semiconductor chip 3 and the first metal member 2 to each other by bonding, thus connecting the semiconductor chip 3 to ground. A mold area 6 is sealed up by a molded resin body 7. The molded resin body 7 serves to protect the semiconductor chip 3 and the metal wires 4, 5 and also to position the first metal member 2 and the second metal members 1 securely relatively to each other.
As shown in FIG. 1(b), each of the transmission lines 8 is formed by the second metal member 1 sandwiched between and spaced from two portions of the first metal members 2 by a distance G within the molded resin body 7 which has a relative dielectric constant .epsilon..sub.r. The characteristic impedance Z of the transmission lines 8 can be designed as follows: First, the electrostatic capacitance C between the second metal member 1 and the two portions of the first metal member 2 which are disposed one on each side of the second metal member 1 is determined according to a mathematical process such as a conformal mapping process or the like. The characteristic impedance Z may be determined according to the following formula (1): EQU z=(.epsilon...mu.).sup.1/2 /C. (1)
where .epsilon. is the dielectric constant of the medium, and .mu. is the magnetic permeability of the medium.
Transmission lines for high-frequency signals in communication devices and transmission devices which employ semiconductor devices of the type described above are designed primarily with a characteristic impedance of 50 .OMEGA. or 75 .OMEGA.. For example, if the molded resin body 7 has a sufficient resin thickness and the relative dielectric constant .epsilon..sub.r is almost 5, then the characteristic impedance Z of the transmission lines 8 may be realized almost 50 .OMEGA. when a lead frame is designed with the thickness "d" of the first metal member 2 and the second metal members 1 being almost 0.2 mm, the distance G between the first metal member 2 and the second metal members 1 being almost 0.45 mm, and the width W of the second metal members 1 being almost 0.4 mm.
In the manufacture of the semiconductor device of the type described above, the cost of facility investments may be minimized by employing a production line for existing semiconductor packages having standardized external forms.
FIGS. 2(a), 2(b), and 2(c) show, in plan, front elevation, and side elevation, respectively, an example of an existing semiconductor package which is standardized. The appearance of the semiconductor package shown in FIGS. 2(a) through 2(c) is a 225-mil. 20-pin SSOP (Shrink Small Outline Package) with a lead pitch P of 0.65 mm and a lead width W of 0.22 mm.
FIG. 3 fragmentarily shows in plan a high-frequency signal transmission line at enlarged scale where the arrangement disclosed in U.S. Pat. No. 5,057,805 is incorporated in the 20-pin SSOP shown in FIGS. 2(a) through 2(c). FIG. 3 also illustrates a conductor of a high-frequency signal transmission line on a packaging substrate in an external circuit. In FIG. 3, references P, M correspond respectively to the lead pitch P and the lead width M shown in FIG. 2(b). A second metal member 1 as a central conductor and first metal members 2 sandwiching the second metal member 1 and serving as two ground conductors jointly make up a high-frequency signal transmission line. The two first metal members 2 sandwiching the second metal member 1 are spaced from each other by a distance L.sub.2. The distance L.sub.2, which may be determined according to (2.times.P-M), is 1.08 mm. The distance L.sub.1 between the centers of the first metal members 2 is 2.times.P=1.30 mm.
Users of the semiconductor device of the type described above use packaging substrates of glass epoxy or Teflon or the like. The packaging substrate has a thickness in the range of from 0.4 mm to 0.8 mm in view of warpage or the like thereof after circuits are patterned thereon. In addition, the high-frequency signal transmission line on the packaging substrate is mainly composed of a microstrip line in order to minimize an area occupied thereby when it is designed.
For example, if a high-frequency signal transmission line of 50 .OMEGA. is produced with a Teflon packaging substrate having a thickness of 0.4 mm, then the conductor 10 of a microstrip line thereof has a width B of almost 1.3 mm. If the 20-pin SSOP which incorporates the arrangement disclosed in U.S. Pat. No. 5,057,805 were mounted on such a packaging substrate, then the distance L.sub.2 (1.08 mm) between the two first metal members 2 sandwiching the second metal member 1 on the package side would be smaller than the width B (1.3 mm) of the conductor 10 which forms the microstrip line on the packaging substrate, resulting in a dimensional mismatch. Therefore, the first metal members 2 of the package would be brought into contact with the conductor 10 on a packaging substrate, causing a short circuit, so that the package could not be mounted on the conductor 10.